I think the biggest problem with Harry Potter and the Cursed Child is that it throws away every degree of character development that was established in the original series and writes it off as meaningless. Literally one difference changes their entire personality, beliefs, background, etc.
Cedric Diggory becoming a death eater due to humiliation sort of goes against everything we know of him as a character. He’s literally the embodiment of a Hufflepuff. Cedric never displayed any half-blood/muggle-born bigotry, he seemed to be liked by all. But I’m expected to believe he turned into a genocidal racist because he got humiliated in front of the school? He was no longer kind and accepting? His whole character before this humiliation meant nothing? This is all it took?
Hermione becomes a bitter professor who seems to enjoy harassing her students because she didn’t end up with Ron? In this timeline, practically everything is the same except her and Ron’s relationship. I’m supposed to believe that this brave, intelligent girl who wanted to be the best and brightest grew up to be like this? Her heroic actions during her preteens/teens meant nothing? Her personal goals meant nothing? Her hunger for activism and equal rights? And she turns into a cruel professor that inflicts the same pain that Snape did to her and other students. Her tears when Snape humiliates her didn’t shape her into being an inspiring professor, at least? She saw the value in students having a teacher like Remus Lupin and that didn’t carry over either?
Then Ron turned out boring and wasn’t funny-Ron without Hermione? Ron was always sarcastic and funny, long before Hermione. So why did this change? A part of Ron’s development in the series is becoming more mature. He stepped up to the plate when he needed to and even after all they went through, he still had his humor and his personality. Ron remained Ron even, after he was tormented by the horcrux and after he lost friends and family. This is who he is. Yes, I think Hermione brought out the best in Ron but with or without her, he still had his personality.
The complexity of these characters sort of gets tossed out the window when all it took was one small thing to change them entirely, to change established development in their character before their reality altered.